1. Field of the Invention
The present invention relates to a braking force control system and method of a vehicle for properly applying braking force to wheels so as to make a stable and smooth turn at cornering.
2. Prior Art
In recent years, due to an increasing concern about vehicle safety, so many techniques for securing safe driving have been developed. Among them, there are several proposed technologies in which when a vehicle turns corners, an optimally controlled braking force is applied to wheels based on data of vehicle driving conditions for accomplishing a stable and smooth cornering of the vehicle.
For example, Japanese Unexamined Patent Application Toku-Kai-Hei 2-70561 discloses a braking force control apparatus for controlling braking force on the basis of a yaw rate or an angular velocity of yawing, i.e., a rotational motion about a vertical axis passing through a center of gravity of the vehicle. In this technology, it is judged whether the vehicle is turning in an under-steer condition or in an over-steer condition with respect to a target yaw rate by comparing an actual yaw rate with the target yaw rate and if the vehicle is in under-steer condition a braking force is applied to inner wheels (wheels on the side of a turning center) to correct under-steer such that a deviation of the actual yaw rate from the target yaw rate is minimized and if in over-steer condition braking force is applied to outer wheels (wheels on the opposite side of a turning center) to correct over-steer.
In this technique, however, since the braking force control is executed so as to bring an actual yaw rate closer to a target yaw rate, if the target yaw rate is established erroneously, then the braking force is also determined erroneously and this erroneous braking force may lead the vehicle to spin or drift-out. For example, on a slippery road, when a vehicle driver turns the steering wheel fully to the lock in order to avoid an obstacle, a target yaw rate produced accordingly is so large that the vehicle itself can not retain a stable running condition. If the braking force control is continued to be applied in this situation, eventually the vehicle will enter into a spin tendency.